3D Printing: Filament Runout Sensor False Trips

This is one for the books. On my Creality Ender 5 Plus 3D Printer I moved the extruder and filament sensor unit from the back of the printer to the front. I tore my rotator cuff and couldn’t reach to the back to load filament (the printer is on a shelf against the wall) and I was scheduled for surgery but wanted to continue printing while recuperating.

Well something went south. Every time I tried to print, after a while the controller thought it ran out of filament when there was still plenty. I’d tell it to continue and it would do it again. Tried different filament sensors, different extension cables, still kept doing it. Started to wonder if the motherboard was having a problem.

Fast forward ~9 weeks to yesterday where I could finally lift and move the printer to troubleshoot. Opened up the controller, reseated the filament sensor connector, closed it up, tried it again. Nope. Still false trips. Didn’t really think that would fix it but process of elimination…

So then I started thinking about the cabling, specifically it’s routing. I routed and bundled the filament sensor’s cable using Ty-wraps running it with two servo cables. Having read about an issue a while back with some BLTouch’s getting interference from heater cartridges (probably due to PWM control) , there even happens to be a setting in Marlin to turn off the heaters during probing, I wondered if the filament sensor was getting noise from the servo cables.

I wasn’t convinced it was a motherboard problem and didn’t have any other ideas at the moment so I unbundled the filament sensor cable from the two servo cables. I let the filament sensor cable hang out in open air and ran a 3+ hour test print. Not really printing though, no filament being pushed to the hotend just a short piece of filament stuck through the sensor. This same test would get false trips before.

No. False. Trips. Just to be sure I ran the 3+ hour print test again. No. False. Trips. It would not make it through this test print process before without tripping…

So the electromagnetic field generated by the current in the servo cables was enough to induce a voltage/current, noise, in the filament sensors cable to make it look like the sensor tripped due to a out of filament condition.

Seriously I was really starting to wonder if the motherboard went bad, but turns out that was not the case. Now I wonder if wrapping the filament cable around a ferrite torrid/ring core right by the motherboard would be enough to eliminate the induced noise.

But I’m not curious enough to try it… Happy Printing!

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